1. Field of the Invention
This invention relates to holography and, more particularly, to a system for generating strip-multiplexed transmission or reflection holograms.
2. Description of the Prior Art
Strip-multiplexed holograms are formed by a plurality of narrow, holographic images recorded on holographic recording medium in a plurality of laterally displaced, adjacent strips. Many of the techniques for making strip-multiplexed holograms have been developed by the Multiplex Company of San Francisco. In their system, an ordinary black and white motion picture film is made of a slowly rotating subject such that the film frames contain views of the subject taken from all angles around the subject. The cinema film is projected by a projector having a laser light source through a large spherical lens and a large cylindrical lens placed directly beyond the spherical lens. The large lenses bring the image to a line focus near the surface of a sheet of holographic recording medium mounted in a film transport. A fan-shaped reference beam, originating at a point directly above the cylindrical lens, is projected onto the holographic recording medium so that the reference beam is superimposed upon the line focus of the image or object beam. The interference pattern formed by the superposition of the image modulated object beam and the reference beam near the line focus of the image is recorded on the sheet of holographic recording medium mounted on an incrementally driven movable platen. Each frame of the motion picture film is thus recorded as a vertical strip hologram, and the full sequence of frames is recorded as a series of adjacent, laterally spaced strip holograms on the holographic recording medium. The resulting composite hologram is viewed by bending it into a cylindrical shape and placing a point source white light such as a small, bright, incandescent bulb, on the axis of the cylinder in a position corresponding to the position of the reference beam source relative to the holographic recording medium when the hologram was recorded. The resulting composite image is an accurate three-dimensional reconstruction of the subject without vertical parallax and in rainbow colors. Geometrically, the hologram formed by this prior art method is approximately equivalent to the white light viewable "rainbow hologram" developed around 1969 by Steve Benton of Polaroid Corporation wherein white-light viewability is obtained by restricting monochromatic viewability to a thin horizontal line as illustrated in U.S. Pat. No. 3,633,989. Further techniques for forming composite holograms which are not viewable in white light, are described in "Optical Holography" by Collier, Burckhardt and Lin, Academic Press 1971. A further development of the above described system of the Multiplex Company allows synthesizing strip-multiplexed holograms without the use of a reference beam. According to this method, a holographic diffraction grating is synthesized by projecting an unmodulated object beam and the reference beam onto the holographic recording medium. The diffraction grating thereby produced is then placed in contact with the holographic recording medium, the reference beam is removed, and the sequence of images (the object beam) is projected as before, each image serving to spatially amplitude-modulate the blank frame holographic diffraction grating which is thus contact printed onto the holographic recording medium in a series of laterally displaced vertical strips. The result is a synthesized holographic three-dimensional image having a quality comparable to images previously made on holographic recording medium using a reference beam.
While the developments of the Multiplex Company represent a significant advance in the state of the art, the system nevertheless exhibits several disadvantages and problems. The large spherical and cylindrical lenses in the system are only suitable for producing one size of synthesized hologram without introducing distortions. To scale up the system to relatively large holograms, for example, larger than 50 centimeters, very large spherical and cylindrical lenses are required which are prohibitively expensive, cumbersome and hence impractical. Furthermore, the lens geometry required by the necessity that the reference beam originate along the axis of the object beam cylindrical lens prohibits certain configurations of the reference beam and certain positions of the reference beam relative to the image.
The input systems for strip-multiplexed holography have generally been limited to cinema film projectors. Therefore, prior art strip-multiplexed hologram systems are not suitable for applications where a short recording time is required such as, for example, in medical diagnoses. No provision has been made in the past for image generation using a video projection kinescope or an image convertor such as the Itek PROM.
Other problems associated with prior strip-multiplexed hologram synthesizers is that they have no provision for producing holograms viewable without distortion in a flat display mode using a point illumination source. The prior systems do not process the object beam to produce higher quality images such as may be produced by using spatial filtering at the Fourier plane for image enhancement or reduction of the effects of grain in the image, or by using electronic image enhancement in connection with a video projector.